Don't waste your time adding a leaf. Buy a set of Timbrens for your truck... it will carry a lot more than adding a leaf and will ride like stock when unloaded (unlike adding a leaf). The great part is there's nothing to adjust (unlike air bags), they are <$200, and take about 20 minutes for any Joe to install. I've tried adding leafs, air bags, and Timbrens, the Timbrens are w/o a doubt the best on the market.
Timbrens and high load rated tires are about the best money you can put in a truck that will be carring heavy loads...
Here's a pic of my truck loaded up almost full with green white oak (a little red in there too). There's close to a cord on there, plus a floor jack, 4 saws, and 30 gals of fuel in the tank. Notice the truck is still sitting almost completely level with the Timbrens installed, the back is probably only down about 3" from stock height.
I've had almost 7000 lbs of stone in the back several times, looks about the same.